1. Field of the Invention
The present invention relates to a method and device for producing a negative electrode of a secondary battery, and a secondary battery using the negative electrode. More particularly, the present invention relates to a method and device for producing a negative electrode of a non-aqueous electrolyte secondary battery. The negative electrode has an active material with large capacity density such as silicon (Si) or Si compounds, and also relates to a method and device for testing the negative electrode.
2. Background Art
Non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries are attracting attention as large-capacity power supplies particularly for portable devices. In recent years, to further enhance the capacity of such batteries, there have been increasing efforts to develop electrode materials (for example, utilizing active materials having large capacity density and reducing additional materials) or to improve structural parts (for example, reducing thickness).
As part of such efforts, it has been tried to make better use of negative electrode active materials containing Si or a Si compound because of their large capacity density and much larger theoretical capacity than graphite. Such a negative electrode active material with large capacity density, however, prevents the battery from having satisfactory charge-discharge cycle characteristics (hereinafter, cycle characteristics) due to the reasons described below. A negative electrode active material with large capacity density generally expands and contracts when the battery is charged and discharged. Therefore, when a negative electrode includes an active material layer formed on a current collector by kneading particulate active material having large capacity density with a conductive agent and a binder, the following may occur. The expansion and contraction of the large capacity density material may destroy the conductive network either in the active material layer or between the active material layer and the current collector. Or, the expansion may induce a compressive stress which causes the active material particles to be broken into fine particles. As a result, there is a decrease in conductivity between the active material particles.
In attempts to overcome the problem of insufficient cycle characteristics, Japanese Patent Unexamined Publication No. 2002-83594 suggests a non-aqueous electrolyte secondary battery using a negative electrode in which Si is sputtered to form a thin film on a current collector such as a copper foil. The active material thin film formed on the current collector is divided into columnar sections by cuts extending in its thickness direction. The columnar sections are spaced with gaps which serve to reduce the stress caused by the expansion and contraction of the active material during charge and discharge. Japanese Patent Unexamined Publication No. 2005-196970, on the other hand, suggests a non-aqueous electrolyte secondary battery using a negative electrode in which a Si-containing active material is formed in the form of inclined columns on a current collector by a gas-phase process. The active material formed in the form of inclined columns on the current collector disperses the stress caused by its expansion into two directions, parallel and vertical to the main surface of the current collector.
When the active material formed on the current collector is a large capacity density material greatly expanding and contracting during charge and discharge, it is necessary to create appropriate gaps between the block-like particles of the active material. However, in the former patent document, the gaps are created by cuts which are formed in the thickness direction of the active material thin film as a result that the film expands and contracts during charge and discharge. Therefore, it is difficult to control the size of the gaps. The latter patent document, on the other hand, does not refer to controlling the gaps between the block-like particles of the active material.